Question: Which of these numbers is prime? ${27,\ 51,\ 80,\ 81,\ 97}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 27 are 1, 3, 9, and 27. The factors of 51 are 1, 3, 17, and 51. The factors of 80 are 1, 2, 4, 5, 8, 10, 16, 20, 40, and 80. The factors of 81 are 1, 3, 9, 27, and 81. The factors of 97 are 1 and 97. Thus, 97 is a prime number.